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United We Stand |
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Brainerd Area Amateur Radio Club, Inc. |
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"Ham Tips" Flexible J-Pole Antenna |
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The J-Pole antenna is just a modified form of a Zepp (Zeppelin) antenna. The radiating portion is a half wavelength. The matching stub is ¼ wavelength long. The feed point is adjusted (prior to soldering it) to achieve the best (lowest) matching SWR.
This particular antenna type is easily built with readily available supplies. If you are intending to use this type of antenna as a permanent base station antenna, you can build it from ½ inch copper pipe. Or, as in this case, you can build it from 300 ohm TV antenna lead-in wire. This flexible antenna is able to be rolled up into a very small volume, so you can easily transport it and use it as a portable base antenna. It can be hung from a tree branch outside, if you’re operating your radio outside, or from a ceiling or doorframe if you’re inside.
The J-Pole does have an advantage, because it’s a half wavelength long, in that it does NOT require a ground plane. ¼ wavelength and 5/8 wavelength antennas WOULD require a ground plane. You will find, however, that J pole antennas tend to “couple” with nearby metallic masses, and I would recommend using a CURRENT CHOKE with this antenna type. You can merely scramble (wind) about 6 loops of your lead in cable – perhaps making these loops about 4-6 inches in diameter. Placement of the choke is not critical, but I usually place it near the base of the antenna.
